Controlled rotor rectifier arrangement for a slip-recovery drive

ABSTRACT

SCR devices replace the usual diodes of the rectifier bridge section of a wound-rotor slip-recovery system motor drive. By controlling the retardation angle of the SCR devices while controlling the line bridge for acceleration from zero speed to normal speed, the rating of the overall system is reduced. By removing the gating signal of the SCR&#39;s while operating at a retardation angle in the range between zero and 90°, operation is immediately brought to a stop in case of an emergency.

REFERENCE TO A COPENDING APPLICATION

The present application is related to patent application Ser. No.229,413 filed concurrently by R. B. Herbert and L. W. Herchenroeder for"Two-Quadrant Operation System For a Slip-Recovery Drive."

BACKGROUND OF THE INVENTION

The invention relates to a wound-rotor slip-recovery motor drive system.The wound-rotor slip-recovery drive has been recognized as veryefficient, rugged and low cost to operate.

Static slip-recovery drives are known and have been found to beadvantageous for many specific applications. Slip-recovery is atechnique generally used with an induction motor of the wound-rotortype. When a variable voltage, variable frequency AC power supply is notavailable for the stator, or such commplexity is not desired, a variablespeed drive can be achieved by controlling the rotor current. Thisapproach which involves a controlled return to the network of the energynot used by the load, has been known with an additional machine as theScherbius system, or as the Kramer system. With the advent of SCR powerswitches, control of the rotor current and slip-recovery have beenpracticed statically. See for instance "Principles of Inverter Circuits"by B. D. Bedford and R. G. Hoft, page 404, FIG. 1154, John Wiley 1964.See also: Proc. IEE, Vol. 110, No. 8, August 1963, "Switching Drive ofInduction Motors" by M. S. Erlicki and Y. Wallach, pp. 1441-1450; IEEEtransactions PAS-85, No. 1, January 1966, "Inverter Motor Speed ControlWith Static Inverters in the Rotor", pp. 76-84; and IEEE transactionsIGA-5, No. 1, January/February, 1969, "Slip Power Recovery in anInduction Motor by the Use of a Thyristor Inverter" by William Shepherdand Jack Stanway, pp. 74-82.

The prior art method consists in rectifying the AC current induced assecondary in the rotor of the motor and in creating in the DC link acounter-electromagnetic-force voltage opposed to the rectified DCvoltage, through coupling of an inverter between the AC power supply andthe DC link. Generally, a transformer is used to couple the AC side ofthe inverter to the network. Thus, SCR control provides speed and torquecontrol of a wound-rotor motor having a stator powered from a constantvoltage-fixed frequency source.

The motor is controlled by controlling the gating angle of thethyristors of the inverter, therefore, the back EMF introduced in the DClink by the inverter, and ultimately the current in the rotor. In otherwords, the inverter controls the DC link voltage, whereby the DC linkcurrent is controlled, thus, the AC current in the wound-rotor.Accordingly, the torque is being controlled. The regulator of such aslip-recovery system includes two nested loops: a current inner loop anda speed outer loop.

Such motor drives are particularly useful for pump and fan drives, sincethese generally operate close to top speed. However, motor operation isnot continuous and there is a speed cycling from zero speed to fullspeed and back. Such abrupt changes in and out of normal operation areextremely unfavorable in several respects.

It is known how to control the operation of a wound-rotor slip-recoverymotor drive to achieve speed and torque control by controlling in adelayed ignition angle mode the thyristors of the inverter coupledbetween the DC link and the main power supply. See for instance:

Shepherd, W. and Stanway, J., "The Polyphase Induction Motor Controlledby Firing Angle Adjustment of Silicon Controlled Rectifiers", IEEEInteract Convention Record 1964, (4) pp. 135-154.

This type of drive, however, raises problems which are to be solved inorder to take full advantage of these inherent qualities. For instance,outside the normal operative range, measures have to be taken for speedadjustment, either in order to smoothly and rapidly increase or reducethe speed or for an abrupt stop in case of an emergency. Moreover, thereis a need to prevent an excessive motor rating normally called foroperation outside the operative range.

In contrast to the aforementioned control approach, it is now proposedto substitute SCR devices for the diodes which constitute the rectifierside in the rotor of the wound-rotor motor, and to so control the SCRdevices at the rectifier side, concurrently with the SCR devices at thepower line side, so as to achieve similar and better results.

The idea of replacing the diodes of the rectifier bridge of the rotor ofa slip-recovery system by SCR devices is found in "Control in PowerElectronics and Electrical Drives" Proceedings of the Second IFACSymposium, Dusseldorf, West Germany, Oct. 3-5, 1977 (Program Press 1978)pp. 559-566 in a paper entitled "Supersynchronous Static ConverterCascade" by P. Zimmermann. The object, in this prior art, is to extendthe operative range of a wound-rotor slip recovery drive abovesynchronous speed, in fact doubling for the same rating, the speed rangeso that the SCR's are working as diodes in the rectifier bridge withinthe subsynchronous range and as inverter devices of the same bridge whenin the supersynchronous range. Moreover, in the aforementioned paper ofZimmermann, the rotor bridge while in the inverter mode is controlled byforced commutation.

An object of the present invention is to maximize the efficiency of awound-rotor slip-recovery drive for speeds below the normal speedoperative range.

It is also an object of the present invention to permit quicklyextinguishing the rotor current of a wound-rotor slip-recovery motordrive in case of an emergency.

Still another object of the present invention is to reduce the overallrating requirement of a wound-rotor slip-recovery system.

These and other objects will appear from the description hereinafter ofthe invention in its preferred embodiments.

SUMMARY OF THE INVENTION

The aforementioned objects are achieved, in accordance with theinvention by providing a static bridge of SCR devices between the ACoutput of the rotor of the induction motor and the DC link to the staticinverter bridge feeding back energy from the rotor to the power supply,and by controlling the firing angle of the SCR's of the rotor bridge insuch a manner during control of the line bridge as a function of theoperative parameters of the AC motor output for speeds lower than themotor operative speed that the DC link voltage at high slip speeds isdecreased, and by removing the gating in the case of an emergency sothat the rotor current can be quickly interrupted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram representing the slip-recovery motor drivesystem according to the invention;

FIGS. 2A-2F represent the voltage of the line bridge of theslip-recovery system of FIG. 1 in the rectifying and inverting domainsat various firing angles;

FIG. 3 shows a current/speed characteristic which is typical of aninduction motor drive coupled to a load of the fan or pump type;

FIG. 4 shows the real and reactive portions of the inverter bridge ofFIG. 1 when the rotor bridge at the opposite side of the DC link isgated at zero-degree retardation so that its thyristors behave like merediodes;

FIGS. 5A, 5B, 5C and 5D illustrate the direction and distribution ofpower between stator rotor and load in situations of synchronous speed;supersynchronous speed, subsynchronous speed and reverse torque;

FIGS. 6A-6G show vectorially the relation between the voltage vector ofthe inverter bridge of FIG. 1 and the voltage vector of the rotor bridgeof FIG. 1 for acceleration from zero speed (6A); acceleration from 50%speed (6B); motoring at 100% speed (6C); deceleration from 100% speed(6D); deceleration from 50% speed (6E); reverse torque at zero speed(6F); and motoring in reverse direction (6G);

FIG. 7 shows the dynamic characteristics of rotor voltage versus speedfor different angles of retardation of the thyristors of the rotorbridges of FIG. 1;

FIG. 8 is a set of curves representing the rotor voltage at the input ofthe rotor bridge of FIG. 1 for six different speeds, respectively;

FIGS. 9A-9C show by three sections the overall organization of a widefrequency gating system typically used according to the invention tocontrol the rotor bridge of FIG. 1;

FIG. 10 is a vectorial representation of the input voltages and of thederived time reference voltage used to define the firing instant, in thecase of the wide frequency gating system of FIGS. 9A-9C; and

FIG. 11 shows the relay with its contacts as can be used in the contextof FIG. 9A.

PRELIMINARY CONSIDERATIONS REGARDING THE GATING OF THYRISTORS IN THECONTEXT OF THE INVENTION

The present invention rests up on the idea that at the entry of the DClink to the line bridge returning slip energy to the network, control ofrectifiers out of the rectifying mode into an inverting mode can providedefinite advantages in the subsynchronous speed range without affectingthe operation at full speed in a fan or pump type of AC motor drive.This is an approach which is different from the one found in PESC-77Record pages 262-267 in an article by S. B. Dewan and J. R. Sylvesterentitled "Thyristor Controlled Rectifying Inverting At Unity PowerFactor" where a chopper is used to vary the DC link voltage and current.Operating on the rectifier itself to modify the DC link is indeedanomalous once it is realized that the AC output of the rotor is athree-phase power supply of widely varying voltage and frequency as themotor passes from zero speed to full speed and conversely. IntroducingSCR devices in the rotor rather than diodes, will allow the bridge tooperate in an inverting mode with particular firing angles, as generallyknown, for instance, from the aforementioned page 404 and FIG. 11.54 of"Principles of Inverter Circuits" by B. D. Bedford and R. G. Hoft.However, the extreme variability of the voltage and frequency input ofsuch inverter makes it unusual and difficult in the present situation tocontrol the SCR devices in a natural mode of commutation.

Accordingly, it is now proposed with a wound-rotor slip-recovery drivesystem having SCR devices connected in a bridge at the AC output of therotor, to improve the gating of the SCR devices so as to enableoperation of the rotor bridge in either a rectifying or an invertingmode relative to the DC link and while controlling the inverter bridgeto run the motor through a range extending between zero speed to fullspeed.

It is also proposed, according to the present invention, to providenatural commutation of the SCR devices of two bridges connected atopposite ends of the DC link of a wound rotor slip-recovery system forall speeds in a range between zero and full speed.

More specifically, the invention allows either electrodynamicacceleration, deceleration or reverse rotation of a wound-rotor motor byestablishing selected gating angles for the thyristors substituted forthe diodes of the bridge normally connected directly on the rotor of aslip-recovery system.

When the rotor thyristors are gated at zero degrees, they operate infact as diodes. However, to start the motor, the thyristors areinitially and typically fired at 60°. As a result, the motor voltage isreduced by half, thus allowing a lower overall rating for the linebridge and transformer equipment which sees the DC link voltage. Thethyristors are thereafter, during acceleration of the motor, fired at anangle of retardation progressively reduced to zero at which time thenormal speed range will have been reached and the thyristors from thenon will operate as diodes. Similarly control of the rotor bridge attypically 155° retardation will allow complete deceleration to zerospeed. Moreover when the rotor thyristors are gated with muchretardation, such as at 155°, the rotor current is caused to be shiftedwith respect to the stator flux, so that the rotor torque becomesnegative. The motor drive will now rotate in the reverse directiontypically at 87% of the rated torque.

Thus, without any basic change to a wound-rotor slip-recovery drivesystem, by merely changing the diodes of the rectifier to be thyristorsand by properly gating such thyristors, a slip-recovery motor drive hasbecome capable of operation in a reversible mode together withcontrolled acceleration or deceleration between zero and full speed.Under such conditions of operation natural commutation of the thyristorsrequires a wide frequency range type of gating system.

Accordingly, the present invention also provides an improved thyristorgating system which is particularly suited for speed control of awound-rotor slip-recovery system through a wide speed range, includingzero speed and maximum speed, either with forward or with reversetorque.

Referring to FIG. 1 the wound-rotor AC motor drive system according tothe invention includes a rotor bridge section RBS comprising SCR derivesT₁ -T₆ connected to phases R, S, T at the output of the rotor RT of aninduction motor having its stator ST supplied with AC current from thethree-phase industrial network L₁, L₂, L₃. In the normal mode, at theoutput, the rotor bridge section RBS generates DC current which passesthrough a DC link DCL which includes a smoothing reactor SR and a shuntSH. The DC link has a positive polarity terminal A and a negativepolarity terminal B. E_(AB) is the voltage in the DC link. Betweenterminals A and B is mounted an inverter bridge INV comprised of threethyristors TH₁ -TH₆. The AC output of the line bridge INV comprisingphases U, V, W is fed into the primary windings W₁ -W₃ of a transformerTNF having secondary windings S₁ -S₃ connected to the main AC powersupply L₁, L₂, L.sub. 3. From shunt SH of the DC link, a signal IArepresentative of the rotor current is fed into a rotor currentregulator CRG set at a current reference value I_(REF) derived via line4 from a speed regulator SRG which is itself controlled by the errorbetween the rotor speed V_(s) from a tachometer T and a speed referencesignal V_(REF).

It is assumed first that thyristors T₁ -T₆ of the rotor bridge sectionRBS are fired at zero degree retardation, thus, that they operate simplyas would diodes. In accordance with the known slip-recovery power suchstatic drive is similar to a DC motor drive. Control of the speed of thewound-rotor motor is achieved by controlling the firing angle of thethyristors TH₁ -TH₆ of the line bridge INV. The basic regulator composedof speed regulator SRG and current regulator CRG establishes the gatingangle applied by gate controller GCI. The back EMF in the DC link AB dueto the bridge in the inverter mode controls the rotor current andtherefore the motor torque, thus motor speed.

The line bridge INV derives rotor power outputted on phases U, V, Wafter rectification of the three phases R, S, T by the rotor bridgesection RBS, and conversion into AC power by the inverter bridge INV.The power outputted on phases U, V, W is pumped back into the AC linesL₁, L₂, L₃ via transformer TNF. By closing each switch TH₁ -TH₆ at theright point in time, the average DC voltage of the DC link betweenterminals AB can be controlled. When gating is such that under naturalcommutation the switch is turned ON at zero degree, as shown by FIG. 2A,the outputted DC voltage E_(AB) is maximum. As the gating angle isretarded the output voltage decreases. This appears from FIGS. 2B, 2C.At 90°, (FIG. 2D) the average output voltage E_(AB) is zero. As thefiring angle determined by gate controller GCI exceeds 90°, being moreand more retarded, the output voltage goes negative and increases inabsolute value until a maximum negative voltage -E_(AB) is reached for180° delay. At negative output voltage, current will flow only if the DCvoltage source as seen from the rotor bridge section RBS is greater thanthe line bridge output. In such case, power will flow from motor rotorside through line bridge INV, in the inverter mode, which converts theDC current into AC current, and by transformer TNF the energy is fedback to the AC lines L₁, L₂, L₃. This is the well-known slip-recoverytechnique of a wound-rotor induction motor. It also appears that theline bridge INV may be either converting, or inverting, depending uponthe direction of power.

If the rotor bridge section RBS is now considered in its full generalitydue to the presence, according to the present invention, of thyristorsT₁ -T₆, rather than diodes as in the prior art, it is observed here thatthe rotor bridge section, depending upon firing angle control of thethyristors, T₁ -T₆, can also be either converting or inverting undernatural commutation by the three input lines R, S, T from the rotor RT.

Referring to FIG. 1, the six thyristors T₁ -T₆ of the rotor bridgesection RBS are gated from six parallel and respective lines 12 whichare the output lines of a wide frequency gating circuit WFG.

The functional relation between rotor bridge RBS and line bridge INVwill now be considered when as shown in FIG. 1, both bridges have SCRdevices gated to be turned ON under natural commutation one by gatepulse generator WFG controlling the retardation angle α_(R) of thyristorT₁ -T₆, the other by gate pulse generator GCI controlling theretardation angle α_(L) of thyristors TH₁ -TH₆. It is assumed, for thepurpose of illustration, that the induction motor is coupled to a loadof the fan or pump type, that is, one having a current/speedcharacteristic such as shown in FIG. 3. Between 0 and 100% (orsynchronous) speed the torque increases from zero as the square of thespeed. In the same instance, FIG. 4 shows the real and reactive portionof the line bridge current when rotor bridge RBS is gated at zero degree(no retardation), e.g., when as stated earlier the thyristors T₁ -T₆behave like diodes.

As generally known, continuous speed control can be obtained by theinsertion through the line bridge INV of a counter EMF in the DC link ofthe rotor circuit, which slip power is recovered through the feedbackloop via transformer TNF. Thus, the inverter back EMF is an AC voltagesource opposing the flow of current. The torque developed isproportional to the real (in-phase) component of the rotor current.Since the current depends upon the difference between the rotor voltageat the particular slip and the inverter voltage, the torque/currentrelationship is constant for a fixed firing angle of the rotor bridge.

A motor drive system can operate from zero speed to synchronous speed,or even at speeds above synchronous speed. The motor is the prime moverbelow synchronous speed, while power is absorbed from the shaft abovesynchronous speed. When the rotor acts as a brake, power is fed back tothe power supply through the inverter INV, which comes from the statorand from the shaft.

FIG. 5A shows the situation at 60% speed, where 40% of the power passesthrough rotor bridge RBS as slip-recovered energy to the line bridgeINV, 60% power is delivered to the load through the shaft.

FIG. 5B shows the situation for supersynchronous speed, with 100% powerfrom the stator, 40% power from rotor (where RBS operates in theinverter mode thanks to thyristors T₁ -T₆), and the shaft delivers 140%power.

FIG. 5C shows the situation when RBS bridge generates power on the rotorat 100% while at synchronous speed, while the stator delivers 100%power, for a total of 200% power on the shaft, thus allowing the sameefficiency with half the normal rating.

FIG. 5D represents reverse torque when the system is regenerating at 60%speed; with 40% power delivered by the rotor through the RBS bridge inthe inverting mode, whereby the stator output collects 100% power.

The operation of the RBS bridge and the line bridge INV under control ofthe firing angle by gate pulse generators WFG (α_(R)) and GCI (α_(L))will be described hereinafter in the context of FIGS. 6A-6G.

Starting at zero speed, the motor drive of FIG. 1 is accelerated up to100% speed by gating bridge RBS at zero degree (rectifying mode) whilebringing TH₁ -TH₆ of line bridge INV from α_(L) =180° down to α_(L) =90°retardation. Actually 180° is not feasible and in practice the maximumretardation will be α_(L) =155°.

FIGS. 6A and 6C show vectorially the relation between the voltage vectorOI of line bridge INV and the voltage vector OR of the rotor bridge RBS(α_(R) =0°) for α_(L) values 155° (a practical angle rather than 180°),117° and 90°, respectively. Vector OR, actually is a fictive vector inamplitude, shown only to indicate an angular position.

α_(L) =155° being the maximum retardation practical with the inverterbridge thyristors, the value of the motor voltage OV, which is 100% atzero speed and 155°, depends upon a proportional factor K=cos α_(L) /cos155°. As shown in FIG. 6B, OV becomes 50% at α_(L) =117° since K=cos117/cos 155=0.5. For 90°, the speed is 100% while K=cos 90°/cos 155°=0.Therefore, the motor voltage is practically nil.

It is assumed now that as the motor is running at synchronous speed(100%), thyristors T₁ -T₆ of rotor bridge RBS are controlled inaccordance with the present invention to decelerate the motor. Firstα_(L) is set somewhat below 90°, namely at 89° as shown in FIG. 6D.Between 180° and 90° line bridge INV is in the inverting mode. Bybringing α_(L) below 90° the line bridge INV becomes operative in theconverting mode, enough to overcome the DC link voltage and forcecurrent into the DC link. At the same time rotor bridge RBS, beingcontrolled at α_(R) =155°, converts the DC link voltage into rotorcurrent, whereby the motor consumes power and establishes a brakingtorque. The value of OV is still very small (source K=cos 89°/cos155°=-(0.02) (FIG. 5D).

Referring to FIG. 6E, deceleration is shown down to nearly 50% speed,e.g., when TH₁ -TH₆ of bridge INV are controlled with α_(L) =65.6retardation. Now K=cos 65.6°/cos 155°=-(0.455). OV has been reduced bynearly one half. Zero speed is reached when α_(L) =30°. FIG. 6Fillustrates the situation when at zero speed α_(L) is brought to 34.4°.Then K=cos 34.4/cos 155=-(0.91). At that moment a reverse torque startsbuilding up, and reverse rotation is reached (FIG. 6G) when α_(L) =25°,since cos 25°/cos 155°=-(1.0). The motor drive is motoring again.

Referring now to FIG. 7, dynamic characteristics of the motor drivesystem according to the present invention are shown for various valuesof α_(R). For the sake of simplicity the characteristics are assumed tobe linear, the rotor voltage V being plotted as a function of speed S.Thus, each slope corresponds to a particular retardation angle α_(R)gating by circuit WFG of FIG. 1 onto thyristors T₁ -T₆ of rotor bridgeRBS. At zero degree, the motoring torque is along line CD, where C isthe ordinate at zero speed, namely 100% of motor voltage and D is theoperating point for zero motor voltage and 100% speed. Typically, lineDF represents motoring torque for α_(R) =60°, e.g., where OC/OF=cos0°/cos 60°=1/0.5=2. Then, F is the ordinate for 50% rotor voltage.Similarly, line DG for 30° retardation intersects the ordinate at G suchthat OC/OG=cos 0°/cos 30°=1/0.86, thus G corresponds to 86% rotorvoltage.

As earlier stated by reference to FIGS. 5A-5C, the motor can beaccelerated from zero speed to 100% speed by controlling α_(L), theretardation angle from circuit GCI of FIG. 1 onto thyristors TH₁ -TH₆ ofline bridge INV from 155° to 90°, while keeping in section RBS α_(R) =0°from circuit WFG. The motoring torque during acceleration is thus alongline CD of FIG. 7.

In accordance with an important feature of the present invention, themotor drive system of FIG. 1 is so arranged that speed control from zeroto maximum speed does not follow line CD which would require maximummotor voltage at zero speed, but rather along FED, namely following FEfrom zero speed to 50% speed as illustrated in FIG. 7, then, ED from 50%speed to 100% speed. In this fashion, the overall rating of the inverteris reduced by half. It follows that E_(AB) in the DC link will also besubstantially reduced, and the rating of transformer TNF will be reducedto the same extent. Line EF is preferred because it maximizes the gainrepresented by area EFC in cutting the rating requirement, while thearea OFED represents a constant rating requirement. Indeed, operativepoint E may be chosen to be closer to D than to C, thereby reducing thenormal slip recovery mode to a narrower speed range and reducing therating further. To achieve a characteristic deviating from line CD, thethyristors T₁ -T₆ are no longer fired always with no retardation as itwould in accordance with FIGS. 6A and 6B if they were mere diodes.Instead, in order to force the operative point M to follow FE, ratherthan CE, while bringing the speed up to 50% (point E) gating pulsegenerator is so controlled as to increase the slope of DF progressivelyas a function of the operative speed S_(M) until DF reaches DE.Therefore, α_(R), the retardation angle of thyristors T₁ -T₆ is broughtdown from 60° (for DF) to 0° (for DE). The control circuit of FIG. 1ensures automatic correction of any error ε appearing with the operativepoint M above (ε>0 if at M1 above DM) or below (ε<0 if at M2 below DM)the assigned level V_(L) corresponding to an operative point on FE,e.g., at half the maximum rotor voltage (OC) in the example chosen.Accordingly, circuit detects the error V_(M) -V_(L) and controls thereference V_(R) applied to WFG in order to bring about a correction inα_(R) whereby the slope of DM matches the speed S_(M) at all times. WhenS_(M) reaches the abcissa S_(e) for 50% speed, the value of α_(R) shouldhave been reduced to zero and thyristors T₁ -T₆ will now on behave likerectifiers. During the overall acceleration process as explained byreference to FIGS. 6A to 6C the retardation angle α_(L) applied bygating pulse generator GCI to thyristors TH₁ -TH₆ of the line bridge INVwill have been changed from an initial 180° (in fact 155°) to the valueα_(L) =90°.

If the load is a fan, or a pump, normal operation will be most of thetime around synchronous speed or somewhat lower. For the sake ofillustration, it is assumed that normal speed range extends from 50% to100% speed, thus along line ED of FIG. 7.

If deceleration is to be effected down to zero speed, as earlier seen byreference to FIGS. 6D, 6E, the thyristors T₁ -T₆ of rotor bridge RBS arecontrolled by circuit WFG so that α_(R) =155°, while circuit GCIestablishes for thyristors TH₁ -TH₆ of inverter bridge INV, aretardation angle α_(L) which initiates at 89° (rather than 90° in orderto overcome the voltage drop of the static devices in the DC link) andis reduced progressively to 60°. As shown in FIG. 7 this corresponds toa slope along DC" (155° ) rather than the maximum slowing downcharacteristic of DC' (180°) which is not feasible in practice. Again,as in the acceleration mode, the present invention offers the advantageof being able to force the operative point G to follow an horizontalline from E' (50% speed) to F' (zero speed), thereby to reduce themaximum rotor voltage required when decelerating the motor.

While control of the retardation angle α_(L) of line bridge INV by gatepulse generator GCI in the context of the present invention does notbring any particular difficulty because INV is coupled throughtransformer TNF to the power supply lines which have a definitefrequency and voltage at all times, control of the retardation angleα_(R) of thyristors T₁ -T₆ is not effected in the conventional mannerfor the following reasons.

FIG. 8 shows the rotor voltage at various speeds, namely (a) to (f) for0%, 20%, 40%, 60%, 80% and 100% speed. At 0% speed the rotor output ismaximum and the frequency is 60 cycles per second if the stator ST issupplied with 60 cycle power from L1, L2, L3. At 20% speed the frequencyis reduced to 48 Hz and the voltage to 80% in magnitude. At 40% speedthe frequency is 36 Hz and the magnitude lowered to 60%, and so on. Itappears that at 80% speed the frequency of the rotor output is merely 12Hz with 20% voltage magnitude, to become at 100% speed zero frequencyand zero magnitude. It is clear that with such variable voltage andvariable frequency input to the bridge across the rotor, control of theSCR devices under natural commutation is anomalous and does not compareto more conventional AC power supplies.

Referring to FIGS. 9A, 9B and 9C, the wide frequency pulse generator WFGof FIG. 1 is shown hooked on the three phases R, S, T of rotor RT.Circuit WFG includes three sections: an acceleration-decelerationcontrol mode section (FIG. 9A); a phase-back control mode section (FIG.9B) establishing a controlled rotor voltage level during a chosen speedrange, like explained earlier by reference to FIG. 7; and a gate driversection (9C) which is responsive to either or both of the two formersections for applying timely gating pulses on six parallel lines G₁ -G₆to the respective control electrodes of SCR devices T₁ -T₆ of the rotorbridge section RBS.

Referring to FIG. 9A, between phase lines R and T a series network ofresistors includes a central potentiometer P₁ inserted between two equalresistors. Similarly between phase lines R and S there is apotentiometer P₂. A potentiometer P₃ is between phase lines S and T. Twoequal resistors (R₁, R'₁) are in series forming a divider between phaseline S and neutral potential line N. Similarly, (R₂, R'₂) are betweenphase line T and line N. (R₃, R'₃) form the divider between phase line Rand neutral line N.

Three operational amplifiers OA₁, OA₂, and OA₃ are associated with therespective phases as follows: Line 21 from potentiometer P₁ goes throughcontacts CT'₁ to junction J₁ to which the middle point of (R₁, R'₁)reaches also, via contacts CT₁. Junction J₁ is connected by line 41 toone input of operational amplifier OA₁. Neutral line N goes by line 51and a resistor to the second input of operational amplifier OA₁. Thesame arrangement prevails for operational amplifier OA₂ (contacts CT'₂,CT₂) to junction J₂ from P₂ and (R₂, R'₂), respectively, with J₂ goingby line 42 to one input of OA₂, as for operational amplifier OA₃(contacts CT'₃, CT₃) to junction J₃ from P₃ and (R₃, R'₃) respectivelywith J₃ going by line 43 to one input of OA₃. Neutral line N isconnected by line 52 and a resistor to the second input of OA₂, whereasit is connected by line 53 and a resistor to the second input of OA₃.The operational amplifiers are mounted with differential inputs andprovides two outputs of equal magnitude but opposite polarities. A relayCT (shown in FIG. 11) controls simultaneously contacts CT₁, CT'₁, CT₂,CT'₂, CT₃, CT'₃ in such a way that, when contacts CT₁, CT₂, CT₃ areopened, contacts CT'₁, CT'₂ and CT'₃ are closed, and conversely.

Operational amplifiers OA₁, OA₂, OA₃ generate on respective pairs oflines (GS₄, GS₁), (GS₆, GS₃) and (GS₂, GS₅) synchronization pulses whichactuate corresponding channels in the gate driver section (FIG. 9C) toapply gating pulses G₁ -G₆ to the respective thyristors T₁ -T₆ as willbe explained by reference to the vectorial diagram of FIG. 10.

FIG. 10 shows the three phase voltage vectors R, S, T which are assumedto turn counter-clockwise, so that when, between phase R and phase T, Rpasses above vector T, as shown in FIG. 10, it is the right instant fornatural commutation of thyristor T₁ of line R for gating at zeroretardation (α_(R) =0) like illustrated in the instances of FIGS. 6A,6B, 6C e.g. when the motor is being accelerated from zero speed tomaximum speed (at the same time thyristors TH₁ -TH₆ at the line side arebeing fired with a retardation angle going from 155° (in practice)initially, to 90° to 100% speed). Timing for such zero retardationfiring angle requires, though, a zero-crossing reference on the timewave reference. However, with phase lines of such widely variablefrequency and voltage (see FIG. 8) the derivation of a time referencewave in the conventional manner (see B. R. Kelly "ThyristorPhase-Controlled Converters and Cycle Converters" 1971, Chapter Nine,pages 229-247) does not work. It is assumed first that zero angleretardation is established by having relay CT close contacts CT'₁, CT'₂and CT'₃, and open contacts CT₁, CT₂ and CT₃ as shown in FIG. 11.Accordingly, it is now proposed, as shown in FIG. 9A first to derive asignal S'_(c) representative of vector S (which is symmetricallydisposed relative to vectors R and T) (FIG. 10). This appears at thedifferential inputs to operational amplifier OA₁. One input is derivedalong the path R₁, R'₁, 31 J1 and 41. Secondly, operational amplifierOA₁ converts such inputted differential signal into two oppositepolarity signals representing the integral of vector S'_(c) ' namely,vectors (+I_(1S)) and (-I_(1S)) at 90° and 180° as shown in FIG. 10.These two output signals are applied via output lines GS₁ and GS₄ of OA₁to thyristors T₁ and T₄ respectively. From a consideration of FIG. 2A,it appears that while retardation of vector S characterizes a cosinecurve symmetrically disposed between curves R and T, the integral of Sis a curve shifted at 90° therefrom. Therefore, a zero-crossing pointdoes exist exactly at time t₁ when, at operative point a the voltage ofphase R comes to exceed the voltage of phase T. This situation thus,must coincide with T₅ being turned OFF and T₁ being turned ON (FIG. 2A).This result is accomplished by the transition signal on line GS₁ at thecross-over point t₁ of ∫S'_(c). Similarly, at operative point b (timet₄) the voltage of phase R comes to exceed the voltage of phase T in thenegative direction (180° later from the position shown by vectors R, S,T in FIG. 2A). Therefore, a transition signal on line GS₄ will causethyristor T₄ to be turned ON while thyristor T₂ will be turning OFF. Thesame can be said of instants t₃ and t₆ with regard to vector T'_(c) (notshown) and +∫T'_(c) and -∫T'_(c) as derived from operational amplifierOA₂. The same observation can also be made for operational amplifier OA₃regarding vector R'_(c) (not shown) and instants t₂ and t₅. The Table,herebelow, summarizes for zero degree retardation the time relation ofthe transition signals at instant t₁ -t₆ of output lines GS₁ -GS₆ withregard to thyristors T₁ -T₆, phase lines R, S, T and operationalamplifiers OA₁ -OA₃.

                  TABLE                                                           ______________________________________                                        Zero Degree Control By WFG                                                    (Contacts CT'.sub.1, CT'.sub.2, CT'.sub.3 closed)                                        t.sub.1                                                                            t.sub.2                                                                              t.sub.3                                                                              t.sub.4                                                                            t.sub.5                                                                            t.sub.6                               ______________________________________                                        >o Polarity                                                                   Thyristor ON                                                                             T.sub.5 T.sub.1                                                                             T.sub.1                                                                             T.sub.3                                                                           T.sub.3                                                                             T.sub.5                                                                           T.sub.5                          Phase of                                                                      conduction T       R     R     S   S     T   T                                <o Polarity                                                                   Thyristor ON                                                                             T.sub.6 T.sub.6                                                                             T.sub.2                                                                             T.sub.2                                                                           T.sub.4                                                                             T.sub.4                                                                           T.sub.6                          Phase of                                                                      conduction S       S     T     T   R     R   S                                Transition                                                                    ∫R'.sub.c                       x                                        (OA.sub.3, GS.sub.5)                                                          Transition                                                                    -∫R'.sub.c     x                                                         (OA.sub.3, GS.sub.2)                                                          Transition                                                                    ∫S'.sub.c                                                                             x                                                                (OA.sub.1, GS.sub.1)                                                          Transition                                                                    -∫S'.sub.c                 x                                             (OA.sub.1, GS.sub.4)                                                          Transition                                                                    ∫T'.sub.c             x                                                  (OA.sub. 2, GS.sub.3)                                                         Transition                                                                    -∫T'.sub.c                           x                                   (OA.sub.2, GS.sub.6)                                                          ______________________________________                                    

It is now assumed that relay CT (FIG. 11) has been manually set, orotherwise actuated, into its second position. Contacts CT'₁, CT'₂ andCT'₃ are now open, while contacts CT₁, CT₂ and CT₃ are closed. In suchcase, as shown by FIG. 9A, each input line (41, 42 or 43) derives asinput for the associated operational amplifier (OA₁, OA₂ or OA₃) asignal representative of the position of the moving arm on thepotentiometer (P₁, P₂ or P₃). Referring to FIG. 10, if each moving armis offset by, say 25°, from its middle position, e.g. half way betweenthe adjacent phase lines, a vector such as S'_(I) is derived from thepotentiometer (P₁) (similarly R'_(I) for P₃ and T'_(I) for P₂ which arenot shown in FIG. 10 for the sake of clarity) which in the chosenexample is at 155° retardation from vector S (zero degree). Therefore,OA₁, OA₂ and OA₃ are now responsive each to 155° retardation, and theintegrated outputs (±∫R'_(I), ±∫S'_(I) and ±∫T'_(I)) define crossoverpoints at ±90° to the associated vector (S'_(I), T'_(I) or R'_(I)).

It is easily understood how the aforegoing Table should be transposedwhen relay CT is in its second state in order to define firing instantssuch as t₁ -t₆ which correspond to 155° retardation for thyristors T₁-T₆ in accordance with signals outputted on lines GS₁ -GS₆ of FIG. 9A.Thus, control mode of circuit WFG (FIG. 1) is now in accordance withFIGS. 6D, 6E, 6F and 6G, depending upon the retardation angle α_(L)imposed by circuit INV of FIG. 1 (α_(L) =89° to start deceleration from100% speed; α_(L) =65.6° when decelerating from 50% speed; α_(L) =34.4°when exerting a reverse torque at zero speed; α_(L) =25° when motoringis effected in the reverse direction).

Referring to FIG. 9C, the gate driver circuit is shown to include sixoperational amplifiers OA₄, one in each of six channels which arecontrolled by a corresponding one of the six output lines GS₁ -GS₆ fromoperation amplifiers OA₁ -OA₃ of FIG 9A. In response to thesynchronizing pulses of GS₁ -GS₆, operational amplifiers G₁ -G₆ providegating pulses, one for each control electrode of thyristors T₁ -T₆,respectively.

It is observed that operational amplifiers OA₁ -OA₃ provide a sufficientand well defined transition signal at times t₁ -t₆, independently fromthe amplitude of vectors (R'_(C), R'_(I)), (S'_(C), S_(I)) or (T'_(C),T'_(I)) and the frequency of rotation. This can best be understood froma consideration of FIG. 8. When the speed goes up, the rotor voltage(R_(C), S_(C), T_(C)) goes down, and also the frequency goes down. Whenthe frequency decreases, the signals out of one of the operationalamplifiers OA₁, OA₂, OA₃ would normally tend to increase. However, theinputted signals also tend to decrease with speed. Therefore, the twoeffects are compensated by the effect of the integration, and theoutputted signal (±∫R_(c), ±∫S_(c), ±∫T_(c)) remains substantiallyconstant. Therefore, the problem caused during acceleration, ordeceleration, by the wide change in voltage and frequency illustrated bycurves of FIG. 8, no longer exists. As a matter of fact, control bycircuit WFG has become possible in a wide frequency range extending froma 60-period down to a few cycles.

Referring to FIGS. 9B, 9C control of the amplifiers OA₄ of section 9C isshown to further include a phase back signal PBS outputted, throughcontacts CT₄ line 60 and potentiometer P₄ by operational amplifier OA₅.Signal PBS is added to the gating signal from lines G₁ -G₆ so that eachgating signal of lines G₁ -G₆ is synchronized with a bias defined by thephase-back signal amplitude. As a result, retardation of thyristors T₁-T₆ can be modified. In the illustrative embodiment of the invention,signal PBS obtained on line 60 results from a ramp generator comprisingoperational amplifier OA₅ mounted as a comparator so that a variablecurrent I_(AB) representative of the voltage V_(AB) derived from thevoltage sensor VS of FIG. 1 is applied at input junction J₅ via line 61,concurrently with a current I_(L) representative of the bias V_(L)derived from a potentiometer P₅ via line 62. Contacts CT₄ shown in FIGS.1 and 9B are part of a shunting loop between output and input. Whencontacts CT₄ are closed they shunt the ramping circuit comprisingcapacitor C₆ and resistor R' associated with the feedback loop ofoperational amplifier OA₅. A diode D₆ is inserted between diode D₄ andthe output of OA₅ in order to prevent the output from going negativewhen -V_(AB) is less than the bias from P₅. In such case, when the DClink voltage is less than the bias from P₅, the phase back signal PBS isclamped to zero by diode D₄. When, however, -V_(AB) exceeds the biaspoint, the phase back signal PBS is able to ramp positive until V_(AB)becomes equal to the bias level. It appears that the operation of thecircuit of FIG. 9B enables the motor drive system to accelerate whileautomatically changing the motoring characteristic from 60° to zerodegree as explained earlier by reference to FIG. 7, e.g., when speed, asdetected on line 61 (FIG. 9B) goes from zero to 50% the limit levelV_(L) (FIG. 7) being determined by the bias from line 62 (FIG. 9B).Accordingly, the operative point M, while speed is changing willsubstantially follow line FE (FIG. 7).

According to another embodiment of the invention, gating of thethyristors T₁ -T₆ of the rotor bridge section RBS (FIG. 1) isdisconnected in the case of an emergency when said rotor bridge isoperative in a converting mode, e.g., when α_(R) lies between zero and90°. To this effect, an emergency relay SW is triggered by an alarmsignal on line 60. When this occurs, thyristors T₁ -T₆ act immediatelyas open interrupters on each line of the bridge, and the motor drivesystem goes to a stop. This is a significant cost reduction since theexpense of an emergency switch breaker on the main lines can bedispensed of.

We claim:
 1. In a slip-recovery wound-rotor motor drive systemincluding: an AC motor having stator and rotor, an AC power line forsupplying electrical energy to said stator and a load driven by saidrotor; the combination of:a first bridge of SCR (semiconductorrectifier) devices naturally commutated by said rotor; a second bridgeof SCR devices naturally commutated by said AC power line; a DC linkbetween said first and second bridges; first means for gating the SCRdevices of said first bridge to establish a retardation angle α_(R) ;second means for gating the SCR devices of said second bridge toestablish a retardation angle α_(L) ; means being provided responsive toan emergency condition in said system for disconnecting said firstgating means thereby to insure protection of said motor drive system.